Growth Direction and Cross‐Sectional Study of Silicon Nanowires

Abstract

Silicon nanowires (SiNWs) have attracted intense interest in recent years [1] because they are not only important for fundamental studies on various size-dependent phenomena, but they are also potential building blocks for nanoscale electronic and optoelectronic devices. [1±2] Various growth methods, including laser ablation, [3±6] thermal evaporation, [6±9] and chemical vapor deposition, [10±11] have been used to synthesize SiNWs. One important target of the synthetic approaches is to achieve a high degree of control of the diameter, growth orientation, and morphology of the SiNWs. A good understanding of the growth mechanism is obviously the key to obtaining SiNWs with desired properties required for various novel applications. While recent efforts have substantially advanced our understanding on the growth mechanisms of SiNWs, [6] many fundamental issues remain unclear. [3] Although SiO 2 -clad SiNWs appear generally circular in crosssection as shown by transmission electron microscopy (TEM) images, the cross-section of the Si crystalline core has been neither observed nor investigated. The cross-sectional study of SiNWs is expected to provide structural information about the boundary between the crystalline core and the oxide sheath, which, in turn, may shed light on the growth mechanism.In the present work, we have developed a unique samplepreparation technique to prepare the cross-sectional samples of SiNWs for TEM examinations. Using this technique, we successfully prepared and studied the cross-sections of SiNWs, which revealed that the Si core is bounded by well-defined facets of low-index crystallographic planes. We also obtained statistical data to elucidate the growth directions of the SiNWs synthesized by the OAG approach. We point out that the developed technique should be generally applicable to preparing the cross-sectional TEM samples of one-dimensional nanomaterials including nanowires and nanotubes.The as-grown SiNWs are randomly oriented with a length of more than 10 lm and a typical diameter of about 20 nm, as shown in Figure 1a. It is clearly revealed that SiNWs have different shapes. Some have a uniform size along the length of COMMUNICATIONS

Orientation‐Controlled Growth of Single‐Crystal Silicon‐Nanowire Arrays

“…[15] As a result, [110] and [112] are the preferred growth directions for small-diameter SiNWs. [14,15] In our experiments, the diameters of the synthesized SiNWs are larger than 50 nm; therefore, most of them should grow along the <111> directions, as was verified by HRTEM. Keeping in mind the fact that nearly all of our SiNWs grow along the <111> axes whether the growth is epitaxial or not, the question we must now consider is why under some conditions VLSE occurs, while other conditions give rise to randomly oriented SiNWs.…”

Oxide‐Assisted Growth of Semiconducting Nanowires

“…SiNWs grown by OAG technique are primarily oriented in the á112ñ and á110ñ directions, and rarely in the á100ñ or á111ñ directions. [22] The cores of SiNWs are bounded by well-defined low-index crystallographic facets with a variety of shapes that can be circular, rectangular, and triangular. We found a correlation between the cross-sectional shape and the growth direction, and proposed a model to explain these findings.…”

Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires

“…78 Unfortunately, the experimentally observed pentagon-shaped nanowires do not appear to be regularly pentagon-shaped but rather square-shaped with one corner diagonally cut off. 79 The advantages of laser ablation as a Si nanowire growth technique are mainly technical simplicity and versatility: technical simplicity, because there is no need for sophisticated gas installations; versatility, because the composition of the nanowires can be varied by simply changing the composition of the laser ablation target. Tang et al, 80 for example, produced phosphorus doped Si nanowires by means of laser ablation.…”